My 4th grade twins took the Explore yesterday too. In the waiting area before the test, while all the other kids were waiting quietly, ds was alternately bouncing off the walls and finding ways to get error messages on his calculator, while dd allayed her boredom with a thick book. They let her bring the book into the exam room too. Afterwards, she said she like the science test best of the 4 because she finished that one more quickly than the others, leaving her more time to read. The only one she ran out of time on was the math - there were a few on the math that seemed unfamiliar to her. Ds was less casual coming out -- I got the impression he struggled a bit compared to his sister, which is consistent with past performance.
They did the practice test a few weeks ago, which was valuable, but this (the actual test yesterday) was their first experience running into time limits on a test like this, so good experience for the future.
A primary goal for the testing was ammunition in working with our public school system. Next year we will be planning the transition to middle school, and from what we've heard, having our own data about their performance level should be very valuable.